A worm-equipped armature shaft, in which a worm is formed on an armature shaft, is used in a motor disclosed in JP11-146603-A, for example. On the armature shaft is fixed a core, on which a coil is wound thereon from a middle portion to a base end portion side, and a commutator side by side. Both end portions interposing the core and the commutator therebetween serve as bearing portions, which are in slide contact with a pair of bearings disposed in the housing. Correspondingly, the leading end portion of the armature shaft is smaller in diameter than a portion to fix the core and the commutator thereon. On the small diameter portion is formed a worm to be engaged with a worm wheel for driving a motor output shaft. In assembling the motor, it is necessary to insert the armature shaft into the bearings from the leading end thereof. An outer diameter of the worm is configured to be smaller than the bearing portions supported by the bearings so that the worm does not come in contact with the bearing in the insertion time.
Conventionally, these worm-equipped armature shaft is formed by working a cylindrical shaft material. Firstly, a cutting process is performed to form the small diameter portion by cutting the outer circumferential face in the leading end portion on which the worm is to be formed. Next, a grinding process is performed from the small diameter portion to an entire of a base end side portion than the small diameter portion to form the outer circumferential face of the bearing portion in high accuracy. Then, by rotating rolled dies and moving them in a radial direction against the small diameter portion processed by the cutting process, the worm is formed. The worm-equipped armature shaft is manufactured in this manner.
However, the surface of the worked portion becomes a coarse surface as conventionally known, and the outer circumferential face of the small diameter portion formed by the cutting process becomes a coarse surface. Thus, when the worm is formed on the small diameter portion, the outer circumferential face of the small diameter portion, on which the worm is to be formed, is the coarse surface, so that a state of the outer circumferential face of the small diameter portion influences the accuracy of the worm, causing an issue to decrease the accuracy of the worm. When the accuracy of the worm is low, the engagement with the worm wheel deteriorates, to cause such issues as a noise occurrence from the engagement portion and low transmission efficiency. Thus, it is considered to increase the accuracy of the worm by grinding the outer circumferential face of the small diameter portion after the cutting process to form the outer circumferential face in an even surface. However, this method increases manufacturing steps and is not a smart way.
Further, an armature shaft is known, which can receive a steel ball in a ball receiving groove (in an installation depressed portion) formed in an axial end of the armature shaft in a depressed manner to bring the steel ball in contact with a plate provided on an end face of a yoke of a motor so that a thrust force acting on the armature shaft is received at the steel ball and the plate (refer to JP-07-033847-B2, for example). Conventionally, the groove for installing the steel ball is formed by a cutting process, specifically, by putting a cutting blade on the axial end of the armature shaft and moving the cutting blade in an axial direction of the shaft in rotating the shaft.
In a case that a manufacturing step to apply a cold forging process to the shaft is included in manufacturing the armature shaft, the groove for installing the steel ball therein is formed on the axial end of the armature shaft in the above-described cutting process after the cold forging process.
Accordingly, the armature shaft is work hardened by a form fluxion in the cold forging process, to cause such an issue that the cutting blade becomes expensive and the life of the cutting blades is shortened, to form the groove for installing the steel ball on the end face of the hardened armature shaft.
In addition, the armature shaft and the steel ball rotates relative to each other in the groove, thus they rotate in contact with each other on the inner face of the groove. However, the groove is formed by the cutting process, so that the surface roughness of the inner face of the groove is relatively large by the cutting lines due to the cutting blades. This causes various malfunctions such as unusual noise occurrence by bumping motions of the armature shaft and the steel ball, uneven wears of the inner face and the steel ball, and rotation transmission loss.
The present invention is achieved to solve the above-described issues. A first object is to provide a manufacturing method of a worm-equipped armature shaft that can enhance an accuracy of a worm without increasing manufacturing steps.
A second object is to provide: a worm-equipped armature shaft having a large accuracy of a worm without increasing manufacturing steps; and a rotary electric machine provided with the worm-equipped armature shaft.
A third object is to provide: a manufacturing method of an armature shaft having an installation depressed portion at its end face to install a thrust-receiving ball therein, the method being capable of easily forming the installation depressed portion and improving a surface roughness of the installation depressed portion, thereby reducing various malfunction occurrences in the installation depressed portion; an armature shaft manufactured by use of the manufacturing method; and a rotary electric machine provided with the armature shaft.